Greenhouse screens are used frequently for energy saving, shading and temperature control. One known type of greenhouse screens comprises a plurality of flexible strips of film material extending in parallel to each other, and which by means of a knitting, warp-knitting or weaving process are interconnected by a yarn framework to form a continuous product, wherein the strips form a major part of the surface area of the product. Such a greenhouse screen is known for example through EP 0 109 951. Other examples of screens of this type are shown in FR 2 071 064, EP 1 342 824, WO 2008/091192 and in WO 2011/096882.
The strips of flexible film material can be of selected materials providing desired properties with respect to reflection and light, moisture and heat transmission.
DE 20 2008 004 181 U1 discloses a two-layer greenhouse screen comprising a standard greenhouse screen as bottom layer and on top of this layer reflective strips that are glued to the screen at certain intervals. This creates a screen which is less drapable. Water transportation through capillary action is prevented in the areas of the yarn framework located between the bottom and top layer strips. Water may be trapped in those areas of the yarn increasing the risk for algae growth. The double-layer structure further has the disadvantage of creating a big bundle when the screen is in rested position.
US 2004/198126 refers to a light-shading sheet for agricultural and horticultural use comprising a light-shading white film and a reinforcement made of textile fabric or a nonwoven fabric. The reinforcement is laminated to the backside of the film. The film may be slitted to from strips that are laminated spaced apart to the reinforcement. The lamination technique used is adhesive or “sandwich lamination”, wherein the latter is defined as a technique of laminating the textile or nonwoven fabric and the white film via an adhesive layer made of lamination resin disposed between the layers.
JP 10327684 A discloses a shading net capable of reflecting strong solar heat and suitable for agriculture, horticulture etc, The shading net comprises tape-shaped yarns of nonwoven fabric, e g of continuous glass fiber filaments, thermally fused to the surface of a net base fabric. The yarns of the net base fabric may be formed by slitting and stretch-orienting a film.
JP 2004154078 A discloses a greenhouse formed by laminating a thermoplastic film on one or both surfaces of a clothlike material. The clothlike material consists of thermoplastic resin wire elements.
JP 2004160812 A discloses a moisture permeable sheet having water barrier properties and used as an agricultural cover material. The sheet comprises a moisture permeable film laminated on a cloth-like material and a porous sheet.
The aim of protected cultivation in greenhouses is to modify the natural environment to increase yield, to improve product quality, to conserve resources, to extend production areas and crop cycles among others. However, the current trend in horticulture is to be more energy efficient by minimizing energy use in all stages while maximizing production yield. This means that the growers tend to insulate the greenhouse as much as possible during the cold winter months to decrease the amount of energy used to heat the greenhouse, but without negatively affecting the production or quality of the crop. The insulation is normally achieved by using one to several layers of greenhouse screens. To maximize energy savings, the top layer of the upper-most screen should have low emissivity, i.e. low ability to emit energy by radiation. This is normally achieved by using aluminum laminate strips knitted into the structure. However, today's state of the art screens also often have part of the yarn framework covering the top layer aluminum, decreasing the energy saving since the yarns are made of plastic materials with high emissivity. Hence, it would be advantageous to have a screen in which the amount of yarn, especially amount of yarn facing upward, is minimized, thereby minimizing the radiation losses.
The lower-most screen in a multilayer installation is usually a transparent screen. For many crops, the rule of thumb is, 1% less light equals 1% less production. This means that light transmission losses in such a screen should be minimized to increase production when the screen is used during daytime. This can be achieved by choosing materials which maximize the light transmission of the screen. The yarn framework usually prevents a significant amount of light from being transmitted through the screen. Hence, a significant improvement of today's state of the art screen is possible if the amount of yarn used to construct the screen could be decreased without losing out on other important properties of the screen such as robustness, water transmission etc.
The water vapor transmission is a very important property of the screen since this is a significant factor controlling the humidity level in the greenhouse. The water vapor transmission is mainly controlled by the width of the strips and the type of yarn used. The yarn should therefore have liquid-transporting capacity by capillary action, in order to be able to absorb and distribute condense water along the screen. Normally, the strip should completely fill the voids between the pillar stitches to maximize the energy saving properties of the screen. In some cases, it is an advantage to have a screen that can transmit more water vapor, for example avoiding having to open the screen (“gapping”) to lower the humidity level in the greenhouse. This could be achieved by using a narrower strip. However, in the state of the art screen of today it is not possible to make the strip narrower since this may cause problems with strips being pulled out of the structure of the screen by wind etc. when the strip is no longer in contact with the yarn that will hold the strip in place by friction forces.
The bundle size is an important feature of the screen. A big bundle of a single screen can cause losses of ˜3% of natural light, hence to maximize production the screen bundle size should be minimized. The bundle size is controlled by the thickness of the film, and the yarn framework that is used.
The robustness of the screen is another important feature. The installation of the screen in the greenhouse is often made by unskilled labor. Hence it is vital to make a screen that can withstand rough handling during installation and use without compromising the excellent properties needed to achieve the right climate for the plant, such as high light transmission, high reflection, low emission, air tightness etc.